1. Field of Invention
The present invention relates generally to micromachining and, more particularly, to a micromachined infrared sensitive pixel that may be used in, for example, an infrared imager.
2. Description of the Background
The human eye can only gather information from the visible light region of the electromagnetic spectrum. Imaging in the infrared (IR) region of the spectrum, however, allows sensing of small differences in temperature in the environment. This ability is highly useful for surveillance and security applications as it enables detection of objects in absolute darkness. Conventional IR imagers have been in use by law enforcement and military agencies. However, conventional IR imager technology relies on semiconductor sensors, which are expensive and bulky due to the necessary cooling to liquid nitrogen temperatures (77 K). Newer uncooled imagers are based on custom materials and are expensive to fabricate.
The applications for low cost infrared imagers range from household security applications to xe2x80x9cnight visionxe2x80x9d aids for night driving. IR sensors with high enough sensitivity (0.1 degree Kelvin) to be suitable for imaging purposes are expensive. Imagers made using conventional semiconductor technologies are very expensive and bulky, as the imagers have to be cooled to the temperature of liquid nitrogen. Newer uncooled imagers are based on custom processes that are expensive to fabricate.
Several research and development groups have created medium-performance IR imaging arrays. One example is the Honeywell uncooled bolometer array. Certain technology spearheaded by Sarnoff Laboratories and Sarcon Microsystems includes the creation of suspended bimorph micro-plates whose thermo-mechanical displacement is detected by measuring the capacitance of the micro-plate to the substrate. The main drawback of that approach is that the designs are difficult to manufacture due to the good control of residual stress.
Other approaches to uncooled IR imagers include the Texas Instrument Approach: An array of 25 xcexcm-75 xcexcm Barium Strontium Titanate (BST) detectors whose polarization and electric constant change with temperature, resulting in a change in capacitor charge as the scene temperature varies. This technology is limited by the ability to obtain good quality, very thin BST films. The best reported Noise Equivalent Difference Temperature (NEDT) for the system was 100 mK (C. Hanson, xe2x80x9cUncooled thermal imaging at Texas Instruments,xe2x80x9d SPIE vol. 2020, Infrared Technology XIX (1993)).
An approach by Honeywell uses an array of micro-bolometers with Vanadium Oxide (VOx) resistors (Temperature Coefficient of Resistance (TCR) 2%/K). The bolometer is suspended by a silicon nitride bridge for thermal isolation from the substrate containing the read out electronics. The commercial products achieve a Noise Equivalent Difference Temperature (NEDT) in the range of 100 mK and have been integrated in video rate and single shot digital cameras. Recent results from Raytheon have reported a 20 mK NEDT for a 2500 xcexcm2 pixel size. This approach is limited by the self-heating of the pixel and 1/f noise. (B. E. Cole et al., xe2x80x9cMonolithic Two-Dimensional Arrays of Micromachined Microstructures for Infrared Applications,xe2x80x9d Proceedings of the IEEE, Vol. 86, no. 8, pp. 1679-1686, 1998, and W. Radford et al., xe2x80x9cMicrobolometer Uncooled Infrared Camera with 20 mK NEDT,xe2x80x9d SPIE Conference on IR Tech. and Applications XXIV, San Diego, Calif., July 1998, pp.636-646.
An approach by Sarnoff Research Center senses capacitance change of a bimetallic element with the substrate. The thermal isolation is designed using a SiC suspension and the bimetallic strip includes SiC and Aluminum. The theoretical value for the NEDT using this technology is 5 mK.
Silicon infrared imagers developed at the University of Michigan use a n+polysilicon and gold thermocouple to measure the temperature difference between the pixel and the substrate. The best reported NEDT for a 200 xcexcm by 650 xcexcm device was 200 mK. These devices have been integrated into a standard CMOS process to achieve a NEDT of 320 mK for a 250 xcexcm by 250 xcexcm device. It is difficult to achieve small pixel size with this approach due to the larger number of conductors in the thermopile that increase the pixel thermal conductivity to the substrate.
Heat balancing CMOS imagers also developed at the University of Michigan use suspended CMOS transistors that are heat balanced to cancel out the incident infrared radiation. The group fabricated a 100 xcexcmxc3x97100 xcexcm pixel and reported a detectivity of 3xc3x97107 cm-(Hz)xe2x88x921/2/W. The thermal isolation between the substrate and the pixel is poor due to the large number of conductors incident on the pixel. The approach is limited by the size of the pixel that cannot be reduced due to etching considerations.
Pyroelectric detectors using PVDF film deposited on CMOS have been demonstrated by Binne et al, xe2x80x9cAn integrated 16xc3x9716 PVDF pyroelectric sensor array,xe2x80x9d IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 47, Issue Nov. 6, 2000, pp. 1413-1420. The main drawback to this approach is that it requires an external mechanical chopper for operation.
A brief summary of the state of the art pixels demonstrated by industry and research groups is summarized in Table 1 below.
Accordingly, there is a need for a low cost imager that has the potential of being utilized in markets and applications that until now were not cost effective. For example, there is a need for an imager for applications such as night driving aids. IR closed circuit cameras could aid surveillance and home security systems. Low cost IR imagers can be used to identify sources of heat leaks in homes and factories leading to energy savings.
According to one embodiment, the present invention is directed to a pixel for an IR sensor. The pixel includes a substrate assembly having a cavity defined by at least one sidewall and a cantilevered beam connected to the substrate assembly and disposed in the cavity. The cantilevered beam includes a first spring portion and a first capacitor plate portion, wherein the first spring portion includes at least two materials having different coefficients of thermal expansion. The pixel further includes a second capacitor plate portion, such that incident IR radiation causes the first spring portion of the cantilevered beam to move laterally relative to the sidewall, thereby creating a variable capacitance between the first capacitor plate portion of the cantilevered beam and the second capacitor plate portion.
According to another embodiment, the pixel includes a substrate assembly, a first cantilevered beam and a second cantilevered beam. The substrate assembly includes a cavity defined by at least one sidewall. The first cantilevered beam is connected to the substrate assembly and disposed in the cavity, and includes a first spring portion and a first capacitor plate portion, wherein the first spring portion includes at least two materials having different coefficients of thermal expansion. The second cantilevered beam is also connected to the substrate assembly and disposed in the cavity, and includes a second spring portion and a second capacitor plate portion, wherein the second spring portion includes at least two materials having different coefficients of thermal expansion, such that incident IR radiation causes the first and second spring portions to move laterally relative to the sidewall thereby creating a variable capacitance between the first and second capacitor plate portions.
According to another embodiment, the present invention is directed to a micromachined structure. The micromachined structure includes a substrate assembly having a cavity defined by at least one sidewall. The micromachined structure also includes a first cantilevered beam connected to the substrate assembly and disposed in the cavity. The first cantilevered beam includes a first spring portion and a first capacitor plate portion, wherein the first spring portion includes at least two materials having different coefficients of thermal expansion. In addition, the micromachined structure includes a second capacitor plate portion, such that incident IR radiation causes the first spring portion of the first cantilevered beam to move laterally relative to the sidewall, thereby creating a variable capacitance between the first capacitor plate portion of the first cantilevered beam and the second capacitor plate portion.
According to another embodiment, the present invention is directed to an infrared (IR) imager including an addressing circuit and a pixel array coupled to the addressing circuit. The pixel array includes a plurality of IR sensitive pixels, wherein each pixel includes first and second cantilevered beams. The first cantilevered beam is connected to a substrate assembly and disposed in a cavity of the substrate assembly, wherein the cavity is defined by a sidewall. The first cantilevered beam includes a first spring portion and a first capacitor plate portion, wherein the first spring portion includes at least two materials having different coefficients of thermal expansion. The second cantilevered beam is also connected to the substrate assembly and disposed in the cavity. The second cantilevered beam includes a second spring portion and a second capacitor plate portion, wherein the second spring portion includes at least two materials having different coefficients of thermal expansion, such that incident IR radiation causes the first and second spring portions to move laterally relative to the sidewall thereby creating a variable capacitance between the first and second capacitor plate portions.